One of the most common types of human diseases throughout the world due to cell abnormalities is cancer, which is also the leading cause of death nowadays. Cancers are fully developed (malignant) tumors with a specific capacity to invade and destroy the underlying mesenchyme, i.e., local invasion. In some cases, invading tumor cells may further penetrate lymphatic vessels or blood vessels newly formed in the tumor and then may be carried to local lymph nodes or even to distant organs where they may produce secondary tumors (metastases). Tumors are usually recognized by the fact that the cells, which may arise from any tissue, are no longer responsive to at least some normal growth controlling mechanisms and hence show abnormal growth. Apart from the cancer, a tumor may merely develop locally and never become malignant, i.e., a benign tumor. Alternatively, cells of a tumor may merely have morphological appearances of cancer cells but remain in their place, i.e., an in situ tumor, although in this case the tumor may sometimes precede a cancer in situ.
There are no absolute methods for diagnosing or assessing the degree of malignancy of tumors. However, among the methods, microscopic examination of tissue is still the most reliable method for routine use. In a pathologic study, tumors can be graded by making an approximate assessment of the degree of structural dedifferentiation (anaplasia) based on histological and cytological criteria by microscopically examining sections thereof. However, on one hand, some cells may have lost their specific structural characters but still retain differentiated biochemical features, while others may still appear differentiated in structure but have lost many normal function attributes. On the other hand, a tumor is not homogeneous and may contain areas with more than one tumor grade, therefore, a developed tumor may consist of a mixed population of cells which may differ in structure, function, growth potential, resistance to drugs or X-rays and ability to invade and metastasize. The two limitations reduce the effectiveness of histological examination of tumors. In another aspect, such an examination by sampling specimens is not suitable for investigations on a large scale.
Many attempts to find absolute markers of malignancy have long been made. Other attempts to identify tumor-specific or tumor-associated proteins, either by direct measurement or by developing specific antibodies to these proteins, are still being made at the moment. They seem to be promising approaches not only in diagnosis but also in providing strategies of destroying cancer cells. A variety of substances wherein the presence or concentrations thereof in vivo may be indicative for certain cancers have been reported, such as oncofetal antigens, e.g., alpha-fetoprotein; serum proteins, e.g., ferritin; enzymes; polyamines; ectopic hormones; cell markers; receptors or tumor-associated viral antigens. However, the most commonly used method of diagnosis of cancers depends on histology rather than any of the above substances. The lack of any absolute markers is a major deficiency in studying cancer.
Recent observations provide some contemplation in searching for the substances intimately associated with carcinogenesis. Cancer is appreciated as a result of multiple gene aberrations which cause both the activation of oncogenes and inactivation of tumor suppressor genes. Further, the differential expression of those critical genes associated with oncogenesis is able to be reflected at the messenger RNA (mRNA) level in cells. For effectively screening the altered ones of interest amongst a great amount of mRNA, a powerful tool, i.e., differential display has been established to identify and isolate a small subset of genes which are differentially expressed between tumorous and normal cells (Liang et al., Cancer Research 52, 6966-6968, 1992).
Human hepatocellular carcinoma (HCC), one of the world's most common cancers, usually develops from chronic inflammatory liver disease via viral infections that induce cirrhosis and exposure to chemical carcinogens (Yu, M. W. et al., Crit. Rev. Oncol. Hematol. 17, 71-91, 1994; Schafer, D. F. et al., Lancet 353, 1253-1257, 1999; Williams, J. H. et al., Am. J. Clin. Nutr. 80, 1106-1122, 2004). In some areas (e.g., China and Africa) HCC is primarily caused by viral infections (HBV, HCV), food contaminated by aflatoxin B1 (AFB1), and other forms of aflatoxin ingestion (Williams, J. H. et al., Am. J. Clin. Nutr. 80, 1106-1122, 2004; Chen, C. J., Hepatology 16, 1150-1155, 1992). Aflatoxin metabolites are secondary products of Aspergillus flavus and Aspergillus parasiticus fungi under hot and humid conditions. These ubiquitous fungi affect such dietary staples as rice, corn, cassaya, nuts, peanuts, chilies, and spices (McLean, M. & Dutton, M. E, Pharmacol. Ther. 65, 163-192, 1995). Chemicals or xenobiotics (such as AFB1) that encounter biologic systems can be altered by metabolic processes. In phase I of the detoxification pathway, cytochrome P450 isoenzymes (induced by polycyclic aromatic hydrocarbons and chlorinated hydrocarbons) add one atom of oxygen to the substrate; bioactivation is an occasional sequela (Hsieh, D. P. H., Elsevier Scientific Publishers, Amsterdam, 1986; Hsieh, D. P. H., Academic, Cambridge, 1987; Aoyama, T. et al., Proc. Natl. Acad. Sci. U.S. A 87, 4790-4793, 1990; Swenson, D. H. et al., Biochem. Biophys. Res. Commun. 60, 1036-1043, 1974). The reactive intermediate aflatoxin B1 8,9-epoxide (produced by CYP isoenzymes, shch as cytochrome P450IA2 and P450IIIA4) is carcinogenic in many animal species; its covalent binding to hepatic DNA has been shown to be a critical step in hepatocarcinogenesis (Forrester, L. M., et al., Proc. Natl. Acad. Sci. U.S. A 87, 8306-8310, 1990; Koser, P. L. et al., J. Biol. Chem. 263, 12584-12595, 1988). Phase II enzymes of primary importance belong to the GST group; these catalyze the conjugation of potentially toxic electrophiles to the GSH tripeptide, generally rendering them non-toxic (Degen, G. H. & Neumann, H. G., Chem. Biol. Interact. 22, 239-255, 1978; Hayes, J. D. et al., Pharmacol. Ther. 50, 443-472, 1991). The reactive aflatoxin B1 8,9-epoxide subsequently attacks and damages DNA. The major AFB1-DNA adduct formed in vivo is AFB1-N7-guanine (Croy, R. G. et. al., Proc. Natl. Acad. Sci. U.S. A 75, 1745-1749, 1978; Kensler, T. W. et al., Cancer Res. 46, 3924-3931, 1986). There are at least two reports indicating that AFB1 binds covalently with DNA and induces G:C to T:A transversions at the third base in codon 249 of p53—considered a hot spot for AFB1 mutagenesis (Bressac, B. et. al., Nature 350, 429-431, 1991; Hsu, I. C. et al., Nature 350, 427-428).
GNMT is an intracellular enzyme which catalyzes the synthesis of sarcosine from glycine. Through this enzyme, glycine receives a methyl group from S-adenosylmethionine (SAM) and becomes sarcosine, which can be subsequently oxidized to become glycine again by sarcosine dehydrogenase. The latter reaction will generate energy and release one carbon unit from SAM. GNMT thus plays a key role in regulating the ratio of SAM to S-adenosylhomocysteine (SAH). The properties of rat liver GNMT, such as its activity being fluctuated and correlated with the level of methionine in the diet and its inducibility with a methionine-rich diet, suggest that it also plays a crucial role in regulating tissue concentration of SAM and metabolism of methionine (Ogawa, H. et al., J. Biol. Chem., 257:3447-3452, 1982). However, GNMT was found to be merely responsible for the metabolism of 20% of total metabolized methionine in vivo (Case et al., J. Nutr. 106: 1721-1736, 1976), but this protein is abundant in liver of mature rats or mice, almost 1% to 3% of the total soluble proteins in liver (Heady et al., J. Biol. Chem., 248:69-72, 1973). Therefore, the GNMT protein may exert other important physiological functions, one of which was found to be identical to a folate-binding protein purified from rat liver cytosol (Cook, R. J. et al., Proc. Natl. Acad. Sci. USA, 81:3631-3634, 1984). Recently, Raha et al. (J. Biol. Chem., 269:5750-5756) proved that GNMT is the 4 S polycyclic aromatic hydrocarbon-binding protein which interacts with 5′-flanking regions of the cytochrome P4501A1 gene (CYP1A1).
Furthermore, as GNMT is the most abundant and efficient methyltransferase in hepatocytes, the activity of GNMT may influence other methyltransferases, e.g., the activity of tRNA methyltransferase can be blocked by GNMT (Kerr et al., J. Biol. Chem., 247:4248-4252, 1972). Results from various laboratories have indicated that lipotropic compounds, such as SAM and its precursors: methionine, choline and betaine, can prevent the development of liver tumors induced by various carcinogens in a rat or mouse model. Due to the findings that GNMT is tightly associated with the SAM level in liver cells and its enzyme activity may be activated by SAM, the GNMT may involve the chemopreventive pathway way of liver cancer (Pascale et al., Anticancer Res., 13:1341-1356, 1993).
It has been reported that diminished GNMT expression levels in both human hepatocellular carcinoma cell lines and tumor tissues (Liu, H. H. et al, J. Biomed. Sci. 10, 87-97, 2003; Chen, Y. M. et al., Int. J. Cancer 75, 787-793, 1998). Human GNMT gene is localized to the 6p12 chromosomal region and characterized its polymorphism (Chen, Y. M. et al., Genomics 66, 43-47, 2000). Genotypic analyses of several human GNMT gene polymorphisms showed a loss of heterozygosity in 36-47% of the genetic markers in hepatocellular carcinoma tissues (Tseng, T. L. et al., Cancer Res. 63, 647-654, 2003). It also reported that GNMT were involved in the benzo(a)pyrene (BaP) detoxification pathway and reduced BPDE-DNA adducts that formed in GNMT-expressing cells (Chen, S. Y. et al., Cancer Res. 64, 3617-3623, 2004).
Previous results indicated that multiple proteins were capable of binding aflatoxin B1 in rat liver cytosol (Taggart, P. et al., Proc. Soc. Exp. Biol. Med. 182, 68-72, 1986). Cytosolic proteins involved in AFB1 binding may have the potential to function in the transport, metabolism and even action of the carcinogen (Dirr, H. W. & Schabort, J. C., Biochem. Int. 14, 297-302, 1987).